dummy load needed for testing CC circuits

I have to be trying to hard to figure this out. What can I use as a substitute for a power led such as an xr-e while testing constant current circuits? Will it be possible to pick a single value 5 watt resistor or diode?

This is the best reference I have found so far. I understand VF can vary on led's and if I was designing a circuit based on voltage it would vary from led to led. But if you already have a circuit to provide constant current (the driver) you don't need another discrete circuit to act as a load for test purposes do you?

If you need a test load and you are worried about damaging an expensive LED, you could put some ordinary diodes in series. A typical diode has a voltage drop of about 0.6 V so if you put six or seven of them in series you will get a drop of 3.6 to 4.2 V. It is easy and cheap to buy rectifying diodes that can handle a 1 or 2 A current without damage. Then put an ammeter in series with the diodes and see if the current is what you expect.

Mr Happy - thats a far better solution than what I've used in the past which was a big resistor submerged in a bowl of water to keep it cool.:thumbsup:

Power diodes are a good solution - compared to a zener, using multiple diodes does spread the heat around, though they'd still get fairly warm with a large current (about 50-60C above ambient for 1N5401 types run at 1A, unless some attempt was made to heatsink them).

The forward voltage of typical power diodes can often be somewhat above 0.6V when passing a reasonable current - possibly closer to 0.8V for currents of a few hundred mA up to an Amp for a 3A diode, and somewhere around 1-1.1V for a diode run at its full rated current.

I guess one other advantage of using regular rectifier diodes is that it would be easy to see how a driver performed with different Vf loads (if that was something that would be worth testing) just by switching in different numbers of diodes.

I must admit I really don't know how to choose a diode for this application. I have a stack of diode's marked 251D. They are from MCM. Specs say 2.5A 1000V. Across a 1.5V battery voltage drop is about .3V each. Across 2x1.5V voltage drop is about .4V each. I see the drop does depend on V in. Will 4~6 of these in series work as my dummy load? And will my circuit act about the same as it would with a power led inline?

P.S. can you give me an idea how to recognize a Schottky diode vs another type of silicon rectifier diode, and what the difference is?

VegasF6 said:

I must admit I really don't know how to choose a diode for this application. I have a stack of diode's marked 251D. They are from MCM. Specs say 2.5A 1000V. Across a 1.5V battery voltage drop is about .3V each. Across 2x1.5V voltage drop is about .4V each. I see the drop does depend on V in. Will 4~6 of these in series work as my dummy load? And will my circuit act about the same as it would with a power led inline?

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That voltage drop seems lower than expectation. The data sheet for a similar diode indicates a forward voltage of about 0.8 V at 1 A. A 1.5 V battery should easily be able to deliver 1 A when shorted through a diode (it might blow up the diode too -- don't do this test without something to limit the current like a small bulb). What does the diode tester on a multimeter say about the forward voltage?

To choose a diode for this application you want a regular power diode like the 251D that you have. The rated current should be comfortably more than you will use, so 2.5 A or more is fine. The reverse breakdown voltage is not a concern in this application, so any value like 1000 V will do.

P.S. can you give me an idea how to recognize a Schottky diode vs another type of silicon rectifier diode, and what the difference is?

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A Schottky diode is a special diode with fast switching action and a low voltage drop. If you have a diode in your hand and you did not specially obtain it as a Schottky diode, the chances are it is not one.

Set for diode check, the 8 different diodes show between 584 and 622, quite a variance. I am assuming then that is between .584 and .622 volts, but it depends on the current load?

I don't know that my first test was done correctly. I touched the cathode of the diode to the cathode of an alkaline AA, then with my meter set for 20V DC touched the black lead of the dmm to the anode of the diode and the red lead to the anode of the battery. Is that a short?

VegasF6 said:

Set for diode check, the 8 different diodes show between 584 and 622, quite a variance. I am assuming then that is between .584 and .622 volts, but it depends on the current load?

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Yes, I would assume those readings are millivolts, since 0.6 V is about right for a silicon diode junction. It is not much variance really, considering the devices are not manufactured for a specific voltage.

I don't know that my first test was done correctly. I touched the cathode of the diode to the cathode of an alkaline AA, then with my meter set for 20V DC touched the black lead of the dmm to the anode of the diode and the red lead to the anode of the battery. Is that a short?

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Ah, that's all right then. It is not a short if you put the voltmeter in series with the diode, but it does explain the low reading. A voltmeter does not pass enough current to properly bias the diode into its conducting region, so the diode therefore appears in the circuit as an extremely high resistance. The way to get an accurate reading is to pass a current of a few mA through the diode and then measure the voltage across it.

If you are measuring 0.3V to 0.4V voltage drop across a diode it may be a germanium diode. They are not as popular a silicon diodes. Their main advantage is the low voltage drop. If you use both silicon and germanium diodes to simulate your load you will be able to get a finer resolution in the value of Vf you are trying to simulate.

At 4V (from power supply this time) I am up to about .6 volts drop. That gave me about 30ma across an unknown 5mm white led. I am playing with this in between painting the living room so it is only getting half my attention
Since I know the source of the diode I don't think it is a germanium, but good point. I have a few others that were given to me that are supposed to be Schottky diodes, I will play with them next. I suppose I will aim for something like 3.4~3.6V at 1 amp, but I haven't used them in a constant current situation yet. Should make testing drivers easier though.

Thanks for the electronics 101, I am trying guys

VegasF6 said:

At 4V (from power supply this time) I am up to about .6 volts drop. That gave me about 30ma across an unknown 5mm white led. I am playing with this in between painting the living room so it is only

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Vegas - good luck on the project. Just a comment on your terminology.
-ma current goes through things... ie measured 30ma going through the 5mm
white led would be correct. And measured .6 volts across it.

I use an old Luxeon emitter that I dropped on the concrete. The dome cracked on it, so it's not good for being in a light anymore. If you happen to have any older emitters that you probably won't be using in lights any more (old Lux IIIs or P-binned crees, for example) that would be an easy solution.